Packaged circuit boards for electronic instruments are typically coated by a moisture-proof insulator film to protect the circuit boards from moisture, electric leakage and dust. Preferably, the moisture-proof insulator films are what are known as conformal coatings, such as acrylic, polyurethane or epoxy synthetic resins dissolved in a volatile solvent. When applied to a clean printed circuit board, a uniform thickness insulative resin film is formed as the solvent evaporates on a continuous basis.
In the past, five principal methods have been used to apply coatings of moisture-proof insulators to printed circuit boards. These methods are discussed in prior U.S. application Ser. No. 06/941,365, filed Dec. 15, 1986, entitled "Method For Applying A Moisture-Proof Insulator Coating on Packaged Circuit Boards", in the name of Takagi Shimada, now U.S. Pat. No. 4,753,819; and, in the continuation-in-part of such application, namely, Ser. No. 07/206,199, filed Jun. 13, 1988, in the name of Takagi Shimada, entitled "Method of Applying A Moisture-Proof Insulative Coating To Printed Circuit Boards Using Triangular or Dovetail-Shaped Liquid Film Emitted From A Flat-Pattern Nozzle", now U.S. Pat. No. 4,880,663. The disclosures of these applications are hereby incorporated by reference in their entirety herein.
As discussed in such applications, the principal methods of applying coatings of moisture-proof insulators to printed circuit boards include:
(a) the immersion method, in which packaged circuit boards are immersed in an immersion tank containing the moisture-proof insulator;
(b) the brush coating method, in which the moisture-proof insulator is applied manually by a brush to the printed circuit board;
(c) the roller method, in which a sheep's wool roll impregnated with a moisture-proof insulator is rolled onto the surface of the printed circuit board to coat it;
(d) the spray method, in which the moisture-proof insulator is applied to the printed circuit board by spraying techniques; and
(e) the slit dye method, in which the moisture-proof insulator is pressurized and extruded from the slit dye to eject a film for coating the printed circuit board surface.
As discussed in each of the above-referenced Shimada patent applications, each of the foregoing methods have certain advantages and disadvantages. For example, all methods except brush coating require masking for those parts of the printed circuit board to be left uncoated. The mounting and removal of masks from the board must be done manually which can create a bottleneck in the mass production of circuit boards. Brush coating, while not requiring masking, is labor-intensive and otherwise unsuitable for mass production.
In order to satisfy demand, the most commonly used insulative coating method employed in mass production is the spraying method. Each of the above-referenced Shimada applications discloses a method of spraying insulative liquid coating material onto a printed circuit board in which a flat pattern nozzle is employed and relative movement is effected between the nozzle and circuit board in a direction transverse to the plane of the flat pattern discharged from the nozzle. The supply of coating material to the nozzle is intermittently interrupted so as to prevent a deposit of liquid coating on regions of the printed circuit board and/or circuit components which are to be left uncoated.
One difficulty associated with the coating method disclosed in the above-referenced Shimada applications is to provide for movement of the spray nozzle or circuit board with respect to one another so that the desired pattern of coating material is obtained at all locations on the board. In mass production, robot arms have often been employed to manipulate the spray guns having spray nozzles which are mounted in a fixed position thereon. These robot arms are capable of moving the spray gun and its spray nozzle in a Z direction, i.e., toward and away from the printed circuit board, and in the X and Y directions, i.e., along the length and width of the circuit board.
In order to ensure that the desired pattern of coating material is applied by the spray nozzle in both the X and Y directions of movement of the spray gun, it has been necessary to reorient the spray nozzle, and thus the spray gun, 90.degree. each time the direction of movement is changed from the X direction to the Y direction or vice versa. Rotation of the entire spray gun requires relatively heavy mechanisms which often are difficult to support on a robot arm and/or which reduce the speed of movement of the robot arm.
Another difficulty associated with prior art circuit board coating devices is that the circuit boards are not necessarily planar because circuit components and the like can protrude from the surface thereof. In some applications, it is desirable to coat the vertical sides or underneath portion of a component carried on a circuit board. This cannot readily be accomplished with robot arms capable of moving a spray gun solely in the X, Y and Z directions.